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4204 CS Computer Graphics OpenGL Shading Language O GL Sh di L Yong Cao g Virginia Tech Reference: Ed Angle, Interactive Computer Graphics, University of New Mexico, class notes Objectives Obj ti Shader applications Vertex shaders Fragment shaders Programming shaders Cg GLSL Vertex Shader A li ti V t Sh d Applications Moving vertices Morphing Wave motion Fractals Lighting More realistic models Cartoon shaders Fragment Shader Applications Per fragment lighting calculations p per vertex lighting g g per fragment lighting Fragment Shader Applications Texture mapping smooth shading g environment mapping bump mapping p pp g Writing Sh d W iti Shaders First programmable shaders were programmed in an assemblyassembly-like manner OpenGL extensions added for vertex and fragment shaders Cg (C for graphics) C-like language for programming Cshaders Works with both OpenGL and DirectX Interface to OpenGL complex OpenGL Shading Language (GLSL) p g g g( ) GLSL OpenGL Shading Language Part of OpenGL 2.0 C High level C-like language New data types Matrices Vectors Samplers OpenGL state available through built-in variables built- Simple Vertex Shader Si l V t Sh d const vec4 red = vec4(1 0 0 0 0 0 1 0); vec4(1.0, 0.0, 0.0, 1.0); void main(void) { gl_Position = gl_ProjectionMatrix *gl_ModelViewMartrix*gl_Vertex; gl_ModelViewMartrix*gl_Vertex; gl_FrontColor = red; } Execution Model E ti Mdl Simple Fragment Program Si lF tP void main(void) { gl_FragColor = gl_FrontColor; } Angel: Interactive Computer Graphics 4E Addison-Wesley 2005 1 0 Execution Model E ti Mdl Angel: Interactive Computer Graphics 4E Addison-Wesley 2005 1 1 Data Types Dt T C types: int, float, bool int, Vectors: float vec2, vec 3, vec4 , , Also int (ivec) and boolean (bvec) ivec) bvec) Matrices: mat2, mat3, mat4 Stored by columns Standard referencing m[row][column] C++ style constructors vec3 a =vec3(1.0, 2.0, 3.0) vec2 b = vec2(a) Pointers Pit There are no pointers in GLSL We can use C structs which can be copied back from functions Because matrices and vectors are basic types they can be passed into and output from GLSL functions, e.g. matrix3 func(matrix3 a) func(matrix3 Qualifiers Q lifi GLSL has many of the same qualifiers such as const as C/C++ Need others due to the nature of the execution model Variables can change Once per primitive Once per vertex Once per fragment At any time in the application Vertex attributes are interpolated by the rasterizer into fragment attributes Attribute Qualifier Att ib t Q lifi Attribute-qualified variables can change at most once Attributeper vertex Cannot be used in fragment shaders g Built in (OpenGL state variables) gl Color gl_Color gl_ModelViewMatrix User defined (in application program) attribute float temperature attribute vec3 velocity Uniform Q lifi d U if Qualified Variables that are constant for an entire primitive C b changed in application outside scope of Can be h di li ti t id f glBegin and glEnd Cannot be changed in shader Used to pass information to shader such as the p bounding box of a primitive Varying Qualified V i Q lifi d Variables that are passed from vertex shader to fragment shader Automatically interpolated by the rasterizer Built in Vertex colors Texture coordinates U User d fi d defined Requires a user defined fragment shader Example: Vertex Shader E l V t Sh d const vec4 red = vec4(1 0 0 0 0 0 1 0); vec4(1.0, 0.0, 0.0, 1.0); varying vec3 color_out; color_out; void main(void) id i ( id) { gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex; gl_ModelViewProjectionMatrix*gl_Vertex; color_out = red; } Required Fragment Shader R i dF t Sh d varying vec3 color out; color_out; color_out out; void main(void) { gl_FragColor = color_out; color_out; } Passing values P i l call by value-return value Variables are copied in Returned values are copied back Rt dl i db k Three possibilities in out inout Operators and Functions O t dF ti Standard C functions Trigonometric Arithmetic Normalize, reflect, length Overloading of vector and matrix types mat4 a; vec4 b c, d 4 b, d; c = b*a; // a column vector stored as a 1d array d = a*b // a ro vector stored as a 1d arra a*b; row ector array Swizzling d S l ti S i li and Selection Can refer to array elements by element using [] or selection (.) operator with x, y, z, w r, g, b, a s, t, p, q a[2] a b, a z, a p are the same a[2], a.b a.z a.p a.b, a.z, Swizzling operator lets us manipulate components vec4 a; a.yz = vec2(1.0, 2.0); Objectives Obj ti Coupling GLSL to Applications Example applications Linking Sh d Li ki Shaders to OpenGL tO GL OpenGL Extensions (With GLEW library) ARB_shader_objects ARB_vertex_shader ARB_fragment_shader g OpenGL 2.0 Almost identical to using extensions Avoids extension suffixes on function names Program Obj t P Object Container for shaders Can contain multiple shaders Other GLSL functions GLuint myProgObj; y gj myProgObj = glCreateProgram(); /* define shader objects here */ glUseProgram(myProgObj); glLinkProgram(myProgObj); Reading Sh d R di a Shader Shader are added to the program object and compiled U Usual method of passing a shader i as a nulll th d f i h d is nullll terminated string using the function glShaderSource lSh d S If the shader is in a file, we can write a reader to convert the file to a string Shader R d Sh d Reader char* readShaderSource(const char* shaderFile) ( ) { struct stat statBuf; FILE* fp = fopen(shaderFile, "r"); char* buf; stat(shaderFile, &statBuf); buf (char*) b f = ( h *) malloc(statBuf.st_size + 1 * sizeof(char)); ll ( t tB f t i i f( h )) fread(buf, 1, statBuf.st_size, fp); buf[statBuf.st_size] = '\0'; fclose(fp); return buf; } Adding V t Sh d Addi a Vertex Shader GLint vShader; GLunit myVertexObj; GLchar vShaderfile[] = my_vertex_shader ; GLchar* vSource = readShaderSource(vShaderFile); dSh d S ( Sh d il ) glShaderSource(myVertexObj, 1, &vertexShaderFile, NULL); myVertexObj = glCreateShader(GL_VERTEX_SHADER); glCompileShader(myVertexObj); glAttachObject(myProgObj, Vertex myVertexObj); Att ib t V t Attributes Vertex attributes are named in the shaders Linker forms a table Application can get index from table and tie it to an application variable Similar process for uniform variables Vertex Att ib t E V t Attribute Example l GLint colorAttr; co o tt colorAttr = glGetAttribLocation(myProgObj, g Get tt b ocat o ( y ogObj, "myColor"); /* myColor is name in shader */ GLfloat color[4]; glVertexAttrib4fv(colorAttrib, color); /* / color is variable in application */ / Uniform V i bl Example U if Variable E l GLint angleParam; angleParam = glGetUniformLocation(myProgObj, "angle"); / /* angle defined in shader */ / /* my_angle set in application */ GLfloat my angle; y_ g my_angle = 5.0 /* or some other value */ glUniform1f(myProgObj, angleParam, my_angle); Vertex Shader A li ti V t Sh d Applications Moving vertices Morphing Wave motion Fractals Lighting More realistic models Cartoon shaders Wave Motion Vertex Shader W M ti V t Sh d uniform fl t ti if float time; uniform float xs, zs; void main() { float s; s = 1.0 + 0.1*sin(xs*time)*sin(zs*time); gl_Vertex.y gl Vertex.y = s*gl Vertex.y; s gl_Vertex.y; gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex; } Particle S t P ti l System uniform vec3 init vel; init_vel; uniform float g, m, t; void main() { vec3 object_pos; object_pos.x = gl_Vertex.x + vel.x*t; object_pos.y = gl_Vertex.y + vel.y*t + g/(2.0*m)*t*t; object_pos.z = gl_Vertex.z + vel.z*t; gl_Position = gl_ModelViewProjectionMatrix* l dli j i i* vec4(object_pos,1); } Modified Phong Vertex Shader I void main(void) /* modified Phong vertex shader (without distance term) */ { float f; ; /* compute normalized normal, light vector, view vector, half-angle vector in eye cordinates */ vec3 norm = normalize(gl_NormalMatrix*gl_Normal); vec3 lightv = normalize(gl_LightSource[0].position -gl_ModelViewMatrix*gl_Vertex); vec3 viewv = -normalize(gl_ModelViewMatrix*gl_Vertex); vec3 h lf = normalize(lightv + norm); 3 halfv li (li ht ) if(dot(lightv, norm) > 0.0) f = 1.0; else f = 0.0; Modified Ph M difi d Phong Vertex Shader II V t Sh d /* compute diffuse, ambient, and specular contributions */ vec4 diffuse = max(0, dot(lightv, norm))*gl_FrontMaterial.diffuse LightSource[0].diffuse; *LightSource[0] diffuse; vec4 ambient = gl_FrontMaterial.ambient*LightSource[0].ambient; vec4 specular = f*gl_FrontMaterial.specular* gl_LightSource[0].specular) *pow(max(0, dot( norm, halfv)), gl_FrontMaterial.shininess); vec3 color = vec3(ambient + diffuse + specular) gl_FrontColor = vec4(color, 1); gl_Position gl_ModelViewProjectionMatrix gl_Vertex; gl Position = gl ModelViewProjectionMatrix*gl Vertex; } Pass Through Fragment Shader /* pass-through fragment shader */ void main(void) id i ( id) { gl_FragColor = gl_FrontColor; } Vertex Shader for per Fragment Lighting Li hti /* vertex shader for per-fragment Phong shading */ varying vec3 normale; varying vec4 positione; void main() { normale = gl_NormalMatrixMatrix*gl_Normal; positione = gl_ModelViewMatrix*gl_Vertex; gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex; } Fragment Shader for Modified Phong Lighting I Ph Li hti varying vec3 normale; varying vec4 positione; void main() { vec3 norm = normalize(normale); vec3 lightv = normalize(gl_LightSource[0].position-positione.xyz); vec3 viewv = normalize(positione); (p ) vec3 halfv = normalize(lightv + viewv); vec4 diffuse = max(0, dot(lightv, viewv)) *gl_FrontMaterial.diffuse*gl_LightSource[0].diffuse; vec4 ambient = gl_FrontMaterial.ambient*gl_LightSource[0].ambient; Fragment Shader for Modified Phong Lighting Li hti II int f; if(dot(lightv, viewv)> 0.0) f =1.0); else f = 0.0; vec3 specular = f* 3 l f*pow(max(0, d t( ( (0 dot(norm, h lf ) halfv), gl_FrontMaterial.shininess) *gl_FrontMaterial.specular*gl_LightSource[0].specular); vec3 color = vec3(ambient + diffuse + specular); gl_FragColor = vec4(color, 1.0); } Vertex vs Fragment Shader per vertex lighting per fragment lighting Samplers S l Provides access to a texture object Defined for 1, 2, and 3 dimensional textures and for cube maps In shader: shader: uniform sampler2D myTexture; myTexture; Vec2 texcoord; texcoord; Vec4 texcolor = texture2D(mytexture, texcoord); texture2D(mytexture, texcoord); In application: texMapLocation = glGetUniformLocation(myProg, myTexture ); glGetUniformLocation(myProg, myTexture ); glUniform1i(texMapLocation, glUniform1i(texMapLocation, 0); /* assigns to texture unit 0 */ Fragment Shader Applications Texture mapping smooth shading environment mapping bump mapping Cube Maps Cb M We can form a cube map texture by defining six 2D texture maps that correspond to the sides of a box Supported by OpenGL Also supported in GLSL through cubemap sampler vec4 texColor = textureCube(mycube, texcoord); textureCube(mycube, texcoord); Texture coordinates must be 3D Environment Map Ei tM Use reflection vector to locate texture in cube map Environment Maps with Shaders Ei tM ith Sh d Environment map usually computed in world coordinates which can differ from object coordinates because of modeling matrix May have to keep track of modeling matrix and pass it shader as a uniform variable Can also use reflection map or refraction map (for example to simulate water) Environment Map Vertex Shader Ei t M V t Sh d uniform mat4 modelMat; uniform mat3 invModelMat; uniform vec4 eyew; void main(void) { vec4 positionw = modelMat*gl_Vertex; vec3 normw = normalize(gl_Normal*invModelMat.xyz); vec3 lightw = normalize(eyew.xyz-positionw.xyz); normalize(eyew xyz-positionw xyz); eyew = reflect(normw, eyew); gl_Position = gl_ModelViewProjectionMatrix*gl_Vertex; } Environment Map Fragment Shader Ei tM F t Sh d /* fragment shader for reflection map */ varying vec3 reflectw; uniform samplerCube MyMap; void main(void) { g gl_FragColor = textureCube(myMap, reflectw); g ( y p, ); } Bump M B Mapping i Perturb normal for each fragment Store perturbation as textures Normalization Maps N li ti M Cube maps can be viewed as lookup tables 1-4 1dimensional variables V t from origin is pointer into table Vector f ii i i t i t t bl Example: store normalized value of vector in the map Same for all points on that vector Use normalization map instead of normalization function sqrt, Lookup replaces sqrt, mults and adds

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